I've been doing T1-T4 for about 5 weeks now with relative easy and have been wanting to try T5. However, I didn't want to try it until I was past what most people would consider the ilvl requirement so I've been aiming for 85 (I did reach 87 today with 2 lucky drops from Coil and still got T4 to do, so I'll be trying T5 tonight).
As long as you're above 80 you'll probably be fine, however the more gear you have the more room for error you have. Being overgeared is almost always better than being undergeared.